Field of the Invention
The present invention concerns a control device for bending compensation control of a multi-roll press.
Description of Background Art
Such a control device is suitable for use in particular in a multi-roll press of a calender intended for the glazing of a continuous thin web, such as a paper web. In a press constructed for this purpose, there are mostly several different rolls placed one above the other in the vertical plane. The topmost one of the rolls and the lowermost one are bending-compensated rolls. In order to increase the line pressure generated by means of the force of gravity of the rolls power units are used at the ends of the topmost roll. Most commonly hydraulic cylinders are used by means of which the press load is controlled. The hydraulic control circuits for varying the bending of the topmost and of the lowermost roll and for adjusting the loading are separate.
In a roll press, the continuous web is glazed so that the web is threaded from the top downwards through the nips between the rolls. The line pressure in the nips increases when going downwards, owing to the weight of the rolls. In order to increase the line pressure in excess of the pressure generated by the force of gravity, power units are placed at the ends of the topmost roll, most commonly hydraulic cylinders, so as to produce and adjust the load.
As the rolls are supported at both ends of their shafts, the extreme rolls in the stack of rolls are bent owing to the load. In order to keep the outer face of the rolls straight and the line pressure uniform, bending-compensated rolls are used as the extreme rolls. Of these compensated rolls, there are several designs of different constructions. However, one common feature of these rolls correction of the deflection dependent on the line pressure, which correcting takes place by adjusting the pressure in the hydraulic system provided between the stationary shaft and the revolving mantle of the roll. This pressure acts upon the mantle either directly or, by means of mobile members, indirectly and keeps the mantle straight and transfers the load to the shaft, which, being resilient, is bent. As a rule, the deflection of the mantle and of the shaft cannot be measured during the operation of the roll press, but for each compensated roll there are predetermined specific adjustment values dependent on the line pressure.
When a multi-roll press is being started and stopped as well as in connection with a paper break, the rolls are either connected with each other or detached from each other by means of a vertical movement of the lowermost roll. The objective of such detaching is to protect the soft-face rolls in the stack of rolls from damage in the case of a break in the web. During the operation of a roll press, a situation often occurs in which the load connected to the topmost roll is changed. In connection with these variations, in particular the mantle of the uppermost compensated roll is deflected, because the hydraulic compensation circuit and the hydraulic load-adjustment circuit operate at different speeds. In the present-day solutions, the operations of the circuits are interconnected so that, when the load is changed, the information on the change passes to the compensation circuit in order to produce a corresponding change. The operation of the compensation circuit is, however, slower than that of the load-adjustment circuit of simpler construction, and in this way unfavourable differences in pressure of short duration arise in the line load in the nip, and so also, in particular in the soft rolls, depressions and changes in the diameter, because of which rolls must be replaced for repair operations.
Most commonly, the hydraulic cylinders that produce the press load are connected together, and the same hydraulic pressure is effective therein. The efficiency of the controlling of the load can be improved further by connecting a separate control circuit to both of the cylinders that generate the press force and by using weight measurement elements in accordance with the Finnish Patent Application No. 810466 at the ends of both the uppermost and the lowermost roll and by providing the adjustment of the line pressures in the uppermost and lowermost nips by means of a processing circuit so that it corresponds to the result of glazing of the paper.
According to the Finnish Patent Application No. 833307, between the press-load control circuit and the compensation circuit of the compensated rolls, a feedback loop is provided, which is arranged so that, in connection with a change in the set value of the press load, it retards the change in the press load so that this change takes place in an appropriate ratio to the speed of control of the compensated rolls.